Carrier Dynamics in Nonfullerene Acceptor Organic Photovoltaics through Ultrafast Spectroscopy.

Nonfullerene acceptors (NFAs) have transformed organic photovoltaics (OPVs) by enabling high efficiencies at small energetic offsets, but rational progress demands a mechanistic view of photoinduced processes across femtosecond-microsecond windows. This review synthesizes ultrafast spectroscopy─transient absorption, pump-push-probe, and two-dimensional electronic spectroscopy─to reveal how NFA molecular design and nanoscale structure govern exciton generation and transport, hybridization of local exciton-charge transfer states, delocalized singlet excitons, hot/endothermic charge separation, and geminate vs nongeminate recombination. Recent studies demonstrate how quadrupole-driven interfacial electrostatics, molecular crystallinity, and energetic disorder contribute to near-barrierless charge separation, long-range delocalization, and strongly non-Langevin carrier dynamics. The review concludes with a roadmap for spatiotemporally resolved, operando studies that integrate ultrafast probes with microscopy to directly correlate local structure with charge carrier dynamics in working NFA-based OPVs.